A paste such as a resin type paste or a calcined type paste which uses a metal filler such as a silver powder or a copper powder are frequently used in the formation of a wiring layer, an electrode, and the like in an electronic device. An electrically conductive film to be a wiring layer, an electrode, or the like is formed by applying or printing a metal filler paste of silver or copper on various kinds of substrates of an electronic device and then subjecting the coated metal filler paste to heat curing or heat calcination.
For example, a resin type electrically conductive paste is composed of a metal filler, a resin, a curing agent, a solvent, and the like, and it is formed into an electrically conductive film by being printed on an electric conductor circuit pattern or a terminal and cured by heating at from 100° C. to 200° C. so as to form a wire or an electrode. In a resin type electrically conductive paste, the thermosetting resin is cured and shrunk by heat, and metal fillers are thus joined by pressure and brought into contact with one another to overlap each other, and as a result, an electrically connected current path is formed. This resin type electrically conductive paste is often used in a substrate using a material susceptible to heat such as a printed wiring board since this paste is treated at a curing temperature of 200° C. or lower.
On the other hand, a calcination type electrically conductive paste is composed of a metal filler, glass, a solvent, and the like, and it is formed into an electrically conductive film by being printed on an electric conductor circuit pattern or a terminal and calcined by heating at from 600° C. to 800° C. so as to form a wire or an electrode. The calcination type electrically conductive paste is treated at a high temperature so that the metal fillers are sintered together to secure the conduction property. This calcination type electrically conductive paste cannot be used in a printed wiring board using a resin material since it is treated at a high temperature for calcination in this manner, but it has an advantage that a low resistance is likely to be obtained since the metal fillers are connected with each other by sintering. Such a calcination type electrically conductive paste is used in, for example, an external electrode of a laminated ceramic capacitor.
Meanwhile, as a metal filler to be used in these resin type electrically conductive paste and calcination type electrically conductive paste, conventionally a silver powder has been often used. However, in recent years, use of a copper powder that is more inexpensive than a silver powder has been preferred because of a steep rise in prices of precious metals and for cost saving.
Here, as the powder of copper and the like to be used as a metal filler, powders having shapes such as a granular shape, a dendritic shape, and a flat plate shape have been often used since electrical conduction occurs as the particles are connected to each other as described above. Particularly, when the particles are evaluated by the size in three directions of length, width, and thickness, the particles having a flat plate shape having a thin thickness have an advantage that the particles can secure a larger contact area between the grains than cubic or spherical particles having a certain thickness and thus can achieve a low resistance, namely, a high electrical conductivity as well as the particles contribute to thinning of the wiring material by a decreased thickness. For this reason, the copper powder having a flat plate shape is particularly suitable for applications of an electrically conductive coating material and an electrically conductive paste which are desired to maintain the electrical conductivity. Incidentally, it is preferable to consider the influence of impurities contained in the copper powder in the case of thinly applying the electrically conductive paste for use.
In order to fabricate such a flat plate-shaped copper powder, for example, Patent Document 1 discloses a method for obtaining a flaky copper powder suitable for a metal filler of an electrically conductive paste. Specifically, a spherical copper powder having an average particle diameter of from 0.5 to 10 μm as a raw material is mechanically processed into a flat plate shape by mechanical energy of a medium filled in the mill by using a ball mill or a vibrating mill.
In addition, Patent Document 2 discloses a copper powder for an electrically conductive paste, specifically, a discoid copper powder capable of obtaining high performance as a copper paste for through holes and external electrodes and a technique relating to a production method thereof.
Specifically, a granular atomized copper powder is put into a medium stirring mill, a steel ball having a diameter of ⅛ to ¼ inch is used as a grinding medium, a fatty acid is added to the copper powder at from 0.5 to 1% by weight, and the granular atomized copper powder is ground in the air or an inert atmosphere to be processed into a flat plate shape.
Furthermore, Patent Document 3 discloses a method for obtaining an electrolytic copper powder which exhibits improved moldability as compared to a conventional electrolytic copper powder and can be molded to have a high strength without developing the dendrite of electrolytic copper powder more than the required extent. Specifically, in order to precipitate an electrolytic copper powder that can be molded to have a high strength by increasing the strength of the electrolytic copper powder itself, the electrolytic copper powder is precipitated by adding one kind or two or more kinds selected from a tungstate salt, a molybdate salt, and a sulfur-containing organic compound to an aqueous solution of copper sulfate of an electrolytic solution for the purpose of miniaturizing the size of the crystallites constituting the electrolytic copper powder.
In any of the methods disclosed in these Patent Documents, the granular copper powder obtained is formed into a flat plate shape by mechanical deformation (processing) using a medium such as a ball, and the size of the flat plate-shaped copper powder obtained by the processing is from 1 to 30 μm as an average particle diameter in the technique of Patent Document 1 and from 7 to 12 μm as an average particle diameter in the technique of Patent Document 3.
Meanwhile, an electrolytic copper powder precipitated in a dendritic shape called a dendritic shape is known, it has a large surface area and exhibits excellent moldability and sinterability as it has a dendritic shape, and it is used as a raw material of oil-retaining bearings and machine parts as a powder metallurgy application. Particularly, in oil-retaining bearings and the like, miniaturization has progressed, and porosity, thinning, and complicated shapes are required along with this.
In order to satisfy these requirements, for example, Patent Document 4 discloses a copper powder for metal powder injection molding having a complicated three-dimensional shape and a high dimensional accuracy and a method for manufacturing an injection molded article using the same. Specifically, it is indicated that the copper powder can be molded to have a high strength since the dendrites of the adjacent electrolytic copper powders are intertwined and firmly linked with one another at the time of compression molding by further developing the dendritic shape. Furthermore, it is described that the fact that the copper powder can have a greater number of contact points as compared to a spherical copper powder since the copper powder has a dendritic shape can be utilized in the case of utilizing the copper powder as a metal filler for an electrically conductive paste or electromagnetic wave shield.
However, in a case of utilizing the dendritic copper powder as a metal filler of an electrically conductive paste or an electromagnetic wave shield, when the metal filler in the resin has a dendritically developed shape, the dendritic copper powders are intertwined with one another to cause aggregation but do not uniformly dispersed in the resin, and viscosity of the paste increases by the aggregation, and this causes a problem in wiring formation by printing. Such a problem is also pointed out, for example, in Patent Document 3.
In this manner, it is not easy to use a dendritic copper powder as a metal filler of an electrically conductive paste and the like and the dendritic copper powder is also a cause of poor improvement in the electrical conductivity of the paste. Incidentally, in order to secure the electrical conductivity, a dendritic shape is more likely to secure the contact points than a granular shape and can secure higher electrical conductivity as an electrically conductive paste or an electromagnetic wave shield.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2005-200734
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2002-15622
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2011-58027
Patent Document 4: Japanese Unexamined Patent Application, Publication No. H09-3510